I am interested in understanding why different groups of vertebrates-animals with backbones-
have very unequal diversity-vastly different numbers of species. For example, there are about
10,000 bird species but only 23 crocodilians. This is an old question, but it is very exciting that
with modern tools allowing us to sequence the base pairs in the DNA of individual genes, we can
get some answers. These DNA sequences evolve, changing at a slow steady rate, so we can use
the cumulative change as a molecular "clock" to estimate the age of different vertebrate groups.
With these tools we tested the hypothesis that groups having the most species today were just the
ones that have been around the longest and have had the most time to diversify. We recently
showed that this hypothesis is false. The evolutionary history of the vertebrates is dominated by
the explosive diversification of relatively young groups, including some (but not all) subgroups of
mammals, birds, and fish. Many classical explanations for species richness don't hold up. For
example, one hypothesis is that bird diversity is linked to the evolution of flight or feathers.
However, we found that the real increase in the rate of bird diversification happened long after
these traits evolved. It is a very exciting time to be in evolutionary biology because at last we can
answer many of our most profound questions about the history of life on earth-questions that
seemed intractable just a few years ago.